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At this stage it is perhaps appropriate to mention that harmonics should be considered at

the design stage and not as an after thought after the system has been installed. Often it

would cost very little to modify the design of a system or a piece of equipment before it is

built or installed so that the harmonic pollution it causes is reduced substantially. Once a

system is installed, it can be very costly to retrofit extra filters and transformers in an

attempt to solve a problem.

7.6.1 Delta connected transformers

Single-phase rectifier bridges employed in switched mode power supplies in PCs and other

electronic equipment, as well as other thyristor controlled devices such as light dimmers,

generate a significant amount of third harmonic and its multiples (i.e. 3rd, 9th, etc.). The use

of a delta-connected transformer provides a circulating path for all the triple harmonics

into the delta windings thus eliminating these from the supply. Harmonic losses in the

windings will then be increased significantly and the transformer must be rated

appropriately.

In the presence of third and triple harmonics, the neutral cable of any star connected load

should be rated significantly above the normal phase ratings (typically by up to 170%)

because of the possibility of large harmonic current flows.

S7: Supply Harmonics

125 EEE8044: Fundamentals

A real life example of what could happen was the supply neutral of a large store fed at LV

from an 800 kVA distribution transformer, with a fluorescent lighting load of 200 kVA,

which was found to be carrying 300 A of zero sequence 3rd harmonic current.

7.6.2 Resonant shunt filters

Passive LC filters tuned to eliminate a particular harmonic are often used to reduce the

level of harmonic components like the 5th and 7th. The filter is usually made up of a

simple LC circuit connected in parallel across the line as shown in the line diagram of

Figure 7.5.

converter

motor

5th harmonic

filter

7th harmonic

filter

incoming high

voltage supply

Fig. 7.5 Tuned 5th and 7th harmonic filters in an industrial application

The resonant frequency fr of the LC branch, given by

LC

f r 2π

= 1

is tuned to the frequency of the harmonic we wish to eliminate so that it provides a path of

low impedance sucking in nearly all of the offending harmonic and leaving the world safer

for freedom, justice and the American Way.

If more than one harmonic is to be eliminated, then a shunt filter must be installed for each

harmonic. Together, the filters form a filter bank connected to the voltage busbar at the

pcc. Care must be taken to ensure that the peak impedances of such an arrangement are

tuned to frequencies between the required harmonic frequencies.

S7: Supply Harmonics

126 EEE8044: Fundamentals

7.6.3 Higher pulse converter circuits

In large converter installations, where harmonics generated by a three-phase converter can

reach unacceptable levels, it is possible to connect two 6-pulse converters in series with

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